1. Field of the Invention
The present invention relates to a measurement method of measuring the shape of a measurement target surface, a measurement apparatus, a non-transitory computer-readable storage medium, and an optical element fabrication method.
2. Description of the Related Art
As a technique for measuring the shape of the surface (measurement target surface) of an optical element such as a mirror or lens, a stitch method of measuring the shapes of a plurality of partial regions obtained by dividing the measurement target surface, and concatenating the shape data of the respective partial regions is well-known. The stitch method can measure a measurement target surface with a large diameter using a measurement apparatus with a small-diameter field of view (measurement region). In terms of the apparatus cost, therefore, the stitch method has the advantage over a case in which a measurement apparatus with a large-diameter field of view is used. Note that since the stitch method moves (for example, tilts, rotates, or translates) the measurement target surface to measure the shape of each partial region, it is necessary to strictly control the positional relationship between the shape data of the respective partial regions.
In the stitch method, therefore, high accuracy is required for the position of a rotation axis about which the measurement target surface rotates and the lateral magnification of the shape data of each partial region. A technique of calibrating them is proposed in U.S. Pat. No. 6,956,657. U.S. Pat. No. 6,956,657 discloses a technique of calibrating alignment errors, systematic errors, positioning errors, and the like so as to reduce errors in regions where a plurality of partial regions overlap each other.
In the conventional technique, however, it is possible to calibrate the position of the rotation axis about which the measurement target surface rotates but the computational load (computation amount) for obtaining the position of the rotation axis is heavy, thereby requiring a long time for the calibration. To reduce the computational load, a high performance processing apparatus or a measurement apparatus for calibration that measures the position of a rotation axis about which a measurement target surface rotates may be used, which, however, unwantedly increases the apparatus cost.